The invention relates to a method and a device for the calibration of an image pick-up device which is sensitive to gravity, for example, because of the fact that it is built into a projecting supporting construction, meaning that the image geometry can be influenced and notably image deformation can be induced by mechanical deformation of the supporting construction. The invention also relates to a method and a device for imaging by means of such an image pick-up device, notably as used in X-ray systems, for example, systems provided with a C-arm. Finally, the invention also relates to an X-ray system provided with such devices.
Image distortions of this kind occur notably in the case of projecting and movable mechanical supporting constructions for such imaging devices, that is, due to mechanical deformations. This problem is encountered notably in X-ray systems in which the image pick-up device is attached to an arm (C-arm) which is rotatable about a patient and at one end of which the image pick-up device is mounted whereas an X-ray source is situated at its other end. Because of the comparatively large dimensions of the arm and the comparatively large weight of these two components, the C-arm may be deformed to such a great and varying extent, depending on its rotary position, that the acquired images are distorted.
WO 00/66971 discloses a device for the measurement of the position and orientation of a body while utilizing means for determining the direction of the gravity vector, as well as a method of correcting X-ray images which have been distorted by the force of gravity and the terrestrial magnetic field. In order to correct an acquired and digitized X-ray image, first the direction of the gravity vector and the position and orientation of the X-ray apparatus are measured and subsequently the distortions of the X-ray image as caused by the deformation of the X-ray apparatus and by the local terrestrial magnetic field are determined and ultimately these distortions in the stored X-ray image are corrected by means of a computer. A significant drawback, however, is then encountered in that the image correction necessitates the presence of the device for determining the exact position and orientation of the X-ray apparatus as well as possibly an additional computer. This additional hardware is disadvantageous notably in the case of mobile X-ray apparatus.
Therefore, it is an object of the invention to provide a method and a device which enable comparatively simple and reliable calibration of an image pickup device in respect of image distortions of the kind set forth.
One object of the invention to provide a method and a device for calibration which is suitable notably for use in C-arm X-ray systems in order to compensate for image distortions which are caused by the mechanical deformations of the C-arm which are due to the force of gravity.
Another object of the invention to provide a method and a device for imaging which is conceived notably for use in conjunction with an image pick-up device calibrated in accordance with the invention and is optionally conceived also for surgical navigation.
Yet another object of the invention to provide an X-ray system which is notably suitable for mobile applications and in which the described gravitational effects can be compensated for without requiring a significant amount of additional hardware such as, for example, a position measuring system. One skilled in the art will recognize that the principles of the invention may be accomplished by meeting one of the recited objects, any combination of the recited objects or non-recited advantages and/or objects that will become apparent to one skilled in the art upon understanding the specification, claims and drawings disclosed herein.
Objects in accordance with accordance with principles of the invention are achieved with a method including the calibration of an image pick-up device which is mounted on a gravity-sensitive supporting construction, the method comprises determining calibration data (calibration base points) in a plurality of selected positions of the supporting construction, which calibration data is suitable for distortion correction of the image acquired by the image pick-up device in the relevant position; determining position data of the supporting construction in the selected positions, said position data being related each time to the direction of the force of gravity, and associating the position data with the calibration data in a lookup table.
An apparatus in accordance with principles of the present invention comprises a device for determining the direction of the force of gravity relative to a position of the supporting construction, and an arithmetic and storage unit for determining position data, related to the direction of the force of gravity, for a plurality of selected positions of the supporting construction and for forming and storing a look-up table whereby the position data is associated with the respective calibration data suitable for the removal of image distortions caused by mechanical deformations in the respective positions.
Objects of the present invention may also be achieved in conformity with a method of imaging by means of an image pick-up device which is attached to a gravity-sensitive supporting construction, the method comprising acquiring an image of an object to be examined in a selected position of the supporting construction, determining position data, related to the direction of the force of gravity, of the supporting construction in the selected position, comparing the position data determined with the position data stored in a look-up table, reading out calibration data stored in the look-up table at position data which correspond at least essentially to the position data determined, and removing the distortions from the acquired image by means of the calibration data read out.
Another apparatus in conformity with aspects of the present invention is a device for carrying out the image forming method, the device comprises a means for determining the direction of the force of gravity relative to the supporting construction, an arithmetic and storage unit for determining position data, related to the direction of the force of gravity, of the supporting construction in order to compare this position data with the position data stored in a look-up table, for reading out calibration data stored at position data in the look-up table which correspond at least essentially to the position data determined, and for removing the distortions from the acquired image of the object to be examined or for distorting the image of an instrument introduced into the object to be examined, by way of the read out and possibly interpolated calibration data, and a display unit for displaying the distortion-corrected image of the object to be examined or for displaying the distorted image of the object to be examined in which the distorted image of the instrument is reproduced.
Advantages of these solutions consist first of all in that essentially higher image qualities can thus be achieved in a controlled manner, that they can be easily carried out and that they are also suitable for use in conjunction with mobile X-ray systems, without the customary position measuring systems being required.
It is also possible to correct distortions which arise due to a curved surface of the entrance window of the image intensifier.
Further advantageous embodiments of a devices in accordance with principles of the present invention include, in one case the calibration data can be calculated by means of a phantom object or, using another approach, by means of a physical model of the mechanical deformation of the supporting construction.
Another embodiment in conformity with aspects of the present invention is particularly suitable in the case of having position data that represents the pivoting angle supporting constructions.
Yet another embodiment in conformity with aspects of the present invention, that is suitable in particular for surgical navigation, includes a method for imaging comprising acquiring an image of an object to be examined in a selected position of the supporting construction and determining position data, related to the direction of the force of gravity, of the supporting construction in the selected position. The method includes comparing the position data determined with the position data stored in a look-up table and reading out calibration data stored in the look-up table at position data which corresponds at least essentially to the position data determined. The method further comprises determining the position and acquiring a (virtual) image of an instrument introduced into the object to be examined, calculating a virtual, distorted image of the instrument, introduced into the object to be examined, by means of the calibration data read out, and reproducing the distorted image of the introduced instrument in the acquired image of the object to be examined.
The following description, claims and accompanying drawings set forth certain illustrative embodiments applying various principles of the present invention. It is to be appreciated that different embodiments applying principles of the invention may take form in various components, steps and arrangements of components and steps. These described embodiments being indicative of but a few of the various ways in which some or all of the principles of the invention may be employed in a method or apparatus. The drawings are only for the purpose of illustrating an embodiment of an apparatus and method applying principles of the present invention and are not to be construed as limiting the present invention.